The narcotic analgesics and their antagonists represent a class of drugs whose effects are believed to be due to interaction with specific tissue receptors. It is not known whether the several effects are mediated through a single receptor or through different receptors. The purpose of this project is to utilize dose-response data (in vivo) for several narcotic responses in order to determine the affinity of these drugs for the receptor or receptors mediating these several kinds of effects. The affinity, measured as either the agonist dissociation constant or the antagonist dissociation constant (pA2), is a quantitative attribute that permits receptor differentiation. The information gained through this study, which utilizes pharmacological (dose-response) methods, will be compared to that obtained by other workers who study the binding of these durgs to tissues, thus permitting anatomical division of receptors. Receptor identficaton may be used to design analgesic drugs with more specific action, thus reducing the untoward effects of the known narcotics.